1. Field of the Invention
The present invention relates to a method for charging a battery built into a radio telephone set for use in a radio telephone system on the basis of the quantity of power consumed by the radio telephone set.
2. Description of the Related Art
A cordless telephone system as one example of a radio telephone system and includes a base unit connected to a telephone line network and a radio telephone set (branch unit) to be coupled to the base unit through a radio link for realizing signal transmission and reception and speech communication with a party. The radio telephone set in the cordless telephone system has a rechargeable battery built therein as its power source. To change the battery, a charger 30 , for example, as exemplified in FIG. 4 has been used.
When it is desired to charge a battery 41 provided in a radio telephone set 40 using the charger 30 shown in FIG. 4, a charging connector 31 provided in the charger 30 is connected to a charging connector 42 provided in the radio telephone set 40.
In the above example, if a transistor 32 is in its OFF state, then a charging current flows from a commercial power source 50 to the battery 41 of the radio telephone set 40 through a D.C. stabilizer power supply 33, a current limiting resistor 34A and a diode 43 for reverse-current prevention, thus charging the battery 41.
In the following explanation, the charging operation when the above transistor 32 is in the OFF state will be referred to as the normal charging operation.
During the charging operation, a voltage detecting circuit 35 detects a voltage across the battery 41 and after detecting a voltage exceeding a preset level, outputs a signal of a logical level "L".
For the purpose of effecting quick charging, a user of the charger closes a switch 36. When the switch 36 is closed a latch circuit 37 is set so that the latch circuit 37 outputs a signal of a logical level "H", and, at the same time, a timer 38 is triggered to start measuring a predetermined time.
At this time, if the voltage of the battery 41 is lower than the above predetermined level, an output signal of the voltage detecting circuit 35 has a logical level of "H". In response to the logic level "H" from the voltage detecting circuit 35, a NAND circuit 39 outputs a signal of a logical level "L" and transistor 32 is turned ON.
When the transistor 32 is turned ON, a current limiting resistor 34B is connected in parallel with the current limiting resistor 34A, so that a charging current larger than in the above normal charging operation flows through the battery 41, thus realizing quick charging of the battery 41.
After the voltage of the battery 41 exceeds the above preset level during the quick charging operation or after the timer 38 completes measuring the predetermined time, the transistor 32 is turned OFF, and the charger is returned to its normal charging mode.
With the aforementioned charger 30, since the quick charging time period is restricted by the function of the voltage detecting circuit 35 or timer 38, the battery 41 is prevented from deteriorated in performance due to the limitless performance of the quick charging operation.
However, the limitation of the timer 38 on the quick charging time involves the following disadvantage.
Since the measuring time of the timer 38 is set based on an average use time of the radio telephone set 40, or in other words, based on an average power consumption of the battery 41 as a reference, in the case where the use time of the radio telephone set 40 becomes much longer than its ordinary use time, the quick charging operation will be undesirably stopped before the battery 41 is sufficiently restored or charged.